410 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



of variation differs much. This is readily evident to the eye on 

 comparing the polygon 8 of Diagram 6 and its correlation table, 

 XIX. (page 466), with polygon 9 (Diagram 6) and its table, XX. 

 In the former the length ranges only from 120 to 176 microns, and 

 the coefficient of variation is 7.003, while in the latter the range of 

 length is from 120 to 220, and the coefficient of variation is 12.767. 

 What is the cause of these great differences in the variation of 

 different groups ? 



4. In different sets belonging to the same general group the 

 correlation between length and breadth differs greatly. Thus, in 

 Table XX. (page 466) the correlation is high and positive, a dift'er- 

 ence in one dimension being accompanied, with much regularity, by 

 a corresponding dift'erence in the other. In Table XXXI. (appen- 

 dix), on the other hand, there is almost no correlation, while in 

 Tables XXIX. and XXXII. the correlation is marked, but negative 

 — an increase in length being associated with a decrease in breadth, 

 and vice versa. What are the causes and significance of these dif- 

 ferences in correlation found in different sets? 



In dealing with these questions, there are three main sets of 

 possible factors to be examined, as follows : 



1. Hereditary Factors. — Some of the factors concerned may be 

 internal and largely independent of the environment — so that the 

 differences in size are hereditary. The existence and nature of such 

 factors form our main problem, but the}^ can be dealt with only 

 after the other factors are investigated. 



2. Growth. — Some of the variations in size, and in proportions, 

 may be due to different stages of growth, so that this matter must 

 be carefully examined. 



3. Environmental Influences. — It appears probable that the dif- 

 ferences in the means, the differences in the range and amount of 

 variation, and in the correlation, may depend partly on the nature 

 of the environment. 



We shall take up in detail these three sets of factors, beginning 

 with growth. 



